Implants with the purpose of restoring missing body functions are becoming more common within medicine and health care. Many different types of implants exist and they are produced from a variety of different materials. In the case of osseointegrated implants, which are devices that are surgically implanted and integrated into living bone, the most commonly occurring materials are metals, such as titanium and titanium alloys, and ceramics, such as zirconium oxide. These materials have mechanical properties similar to the replaced bone tissue.
For the implant to function successfully, a good anchoring of the implant is required. This anchoring can be a combination of mechanical attachment and physico-chemical bonding and is dependent on both the micro structure of the implant surface (see, e.g., Wennerberg et al., J. Biomed. Mater. Res. 30 (1996) 251-260; U.S. Pat. No. 4,330,891 to Branemark et al.; and U.S. Pat. No. 6,689,170 to Larsson et al.), as well as the surface chemistry (U.S. Pat. No. 5,571,188 to Ellingsen et al.; R. G. T. Geesink, Clin. Orthop. 261 (1990) 39-58; Jansen, et al., Mater. Res., 25 (1991) 973-989; Bauer, et al., J. Bone Joint Surg., 73A (1991) 1439-1452; Rashmir-Raven et al. J. Appl. Biomater. 6 (1995) 237-242). In order to increase the osseointegration, coatings possessing enhanced osseointegrating properties have been developed. These coatings create an interface between the living tissue and the implant enabling faster and better osseointegration. One common family of coating materials is calcium phosphates and especially the member hydroxyapatite (HA). HA resembles the mineral found in bone and teeth, is chemically stable, and is known to be one of the few materials that are bio-active, meaning it has the property of initiating a physico-chemical bond between the implant material and the surrounding tissue.
Several coating methods exist for deposition of calcium phosphates onto substrates, which includes thermal plasma spray, sputtering, electrochemical deposition and nanoparticle deposition through immersions into particle dispersions. During the thermal plasma spray process, plasma is produced by letting an electric arc pass through a stream of mixed gases. This process results in partial melting of mineral feedstock, which is hurled with a relatively high force at the substrate, for example an implant. This method may, due to the high temperatures involved, affect the crystallinity of the feedstock, creating a mixture of polymorphs as well as amorphous materials. Traditionally, the plasma spray method produces relatively thick coating layers, usually several micrometers, which can create adherence problems between the substrate and the coating, which in turn may lead to poor osseointegration (Cheang, P. and Khor, K. A. Biomaterials 1996, 17, 537; Groot et al. Biomedical. Mater. Res. 1987, 21, 1375; Story, B. and Burgess, A., S. Calcitek: USA, 1998; and Zyman et al. Biomaterials 1993, 14, 225). The thermal plasma spray technique can be used for producing thinner layers, in the order of 100 nanometers (U.S. Pat. No. 5,543,019), but the problems due to the high temperatures still exists.
Sputtering is used to apply sputtered coatings, which are usually non stoichiometric and amorphous, which can cause severe problems with adhesion and a too high coating dissolution rate when implanted. Furthermore, the sputtering method is seldom used due to its low effectiveness and high costs (Massaro et al., J. Biomedical Materials Research, 58 (6): 651-657 Dec. 5, 2001). The electrochemical deposition technique is relatively inexpensive, but problems can exist with gas formation during the deposition, resulting in problems with cracking and rupture of the coating. The nanoparticle deposition method uses immersions into particle dispersions. This method results in discrete adsorption of nanoparticles using a one step process in which nanoparticles are deposited on an implant surface where the method requires the surface to be pretreated by roughening at least a portion of the implant surface to produce at least a microscale roughened surface (WO 2008/051555) or as a coating using a two step process involving a pre-treatment of the substrate using silanes (U.S. Patent Appl. No. 2004/0249472). This method demands high quality particle dispersions of well defined nanoparticles, well separated in solutions possessing high wettability of the implant surface.
The nanoparticle deposition methods have been shown to effectively increase the osseointegration, but there are several limitations to these methods, such as the need for the addition of silicon chemicals (aminopropyltriethoxysilane) as well as limitations when controlling the deposition of higher amounts than discrete nanoparticles, as in the case when a layer having a particular thickness is desired. There are several other techniques that are described in the literature, including biomimetic methods where simulated body fluids are utilized to form HA coatings (Wei-Qi Yan et al., Biomaterials 1997 (18) 1185-1190), but today it is believed that only the plasma spray and nanoparticle deposition techniques are used commercially. Problems utilizing these above described as well as other techniques not described are plentiful, especially due to that only thick layers can be applied (several μm) leading to problems with adhesion to the substrate and problems with coating objects having complicated shapes. Several of the used or tested techniques also create locally high temperatures, giving amorphous HA instead of the wanted crystalline apatite form. Accordingly, there is a need for new coating methods for the depositions of crystalline nanoparticles, in particular, HA nanoparticles onto surfaces.
Embodiments of the present invention provide methods and systems that can control application of a nanoparticle layer onto micro-rough implant surfaces and associated implants.